U.S. Navy research into VLF/LF propagation phenomena has indicated significant advantage to recording electric field variation aboard in flight aircraft as it varies its distance or azimuth from the transmitter generating the signal. The horizontal electric field under certain propagation conditions will exceed the magnitude of the vertical component at aircraft altitudes even when excited by a vertical radiator. Furthermore, elevated and predominently horizontal radiators can excite higher horizontal fields than vertical radiators.
Data of the variation of signal with ranges or azimuth has been used to help determine the important properties of the propagation medium. Most data obtained in the past has been obtained aboard specially instrumented aircraft whose mission, at the time, was devoted to this purpose and which have had special receiving antennas installed aboard them. However, use of an aircraft with the permanent antenna installation for this purpose only is a relatively costly way of collecting data or receiving signals aboard an aircraft. In order to correctly evaluate and predict coverage from transmitters, it is desirable under certain circumstances, to collect large volumes of data or repeat a measurement on many different days to obtain day-to-day variation of the signal.